The following relates generally to nuclear imaging detector calibration for system such as positron emission tomography (PET) or single photon emission tomography (SPET). It finds particular application in conjunction with energy calibration of a digital PET (DPET) detector and will be described with particular reference thereto. However, it is to be understood that it also finds application in other usage scenarios and is not necessarily limited to the aforementioned application.
In nuclear medicine, radiation events may be detected by scintillators viewed by photodetectors. Using PET as an example, a 511 keV gamma ray characteristic of a positron-electron annihilation event is absorbed by a scintillator crystal which generates a flash (i.e. scintillation) of light. The photodetectors generate a count of photons (in the case of DPET, or a detector electric current in the case of photomultiplier detectors) and the summed photon count or electric current represents the energy of the detected 511 keV gamma ray. Raw mode data of the radiation events includes the position, energy, and timestamp of each detected radiation event. These raw data are first filtered by an energy window, e.g. 511 keV energy windowing for PET, followed by coincidence detection (for PET). Performing the energy windowing before the coincidence detection advantageously greatly reduces the number of events that must be correlated by the coincidence detection.
At a low level, particle energy is measured in terms of the summed photon count or current generated by the photodetector in response to a scintillation. At a higher level, the output is a particle energy value for a detected particle. However, because of individual variations amongst detectors and/or detector electronics, the energy value output by a given detector pixel may differ from the true particle energy by some amount. This variation is corrected by energy calibration. In a typical energy calibration approach, a radioactive calibration source emitting at the energy of interest (511 keV for PET) is loaded into the imaging system and raw data are collected for the calibration source. These raw data are sorted into an event count-versus energy histogram (event-energy histogram). Since the calibration source is designed to have a strong emission at 511 keV, the event-energy histogram should provide a large peak for 511 keV. Any deviation from this 511 keV energy is corrected by multiplying the energy by an energy correction factor. Because of detector non-linearity, this correction factor may be different for different particle energy ranges. One specification of PET detectors is energy resolution, which characterizes how well a detector rejects (e.g., filters) scatter events. The better the scatter rejection capability of a detector, the higher the contrast of the generated images. By way of illustration, energy resolution is an important parameter for DPET systems used to perform quantitative analysis of a treatment's effectiveness over time. A smaller energy resolution helps to keep scatter events away from the true activity distribution and, therefore, improves the accuracy of standardized uptake value (SUV). Calibration of DPET detectors is important to improving energy resolution.
The following provides a new and improved systems and methods which overcome the above-referenced problems and others.